Corrective Action Decision Document/ Corrective Action Plan for Corrective Action Unit 443: Central Nevada Test Area-Subsurface Central Nevada Test Area, Nevada, Rev. No. 0

This Corrective Action Decision Document/Corrective Action Plan (CADD/CAP) has been prepared for the subsurface at the Central Nevada Test Area (CNTA) Corrective Action Unit (CAU) 443, CNTA - Subsurface, Nevada, in accordance with the ''Federal Facility Agreement and Consent Order'' (FFACO) (1996). CAU 443 is located in Hot Creek Valley in Nye County, Nevada, north of U.S. Highway 6, about 48 kilometers north of Warm Springs, Nevada. The CADD/CAP combines the decision document (CADD) with the corrective action plan (CAP) and provides or references the specific information necessary to recommend corrective actions for the UC-1 Cavity (Corrective Action Site 58-57-001) at CAU 443, as provided in the FFACO. The purpose of the CADD portion of the document (Section 1.0 to Section 4.0) is to identify and provide a rationale for the selection of a recommended corrective action alternative for the subsurface at CNTA. To achieve this, the following tasks were required: (1) Develop corrective action objectives; (2) Identify corrective action alternative screening criteria; (3) Develop corrective action alternatives; (4) Perform detailed and comparative evaluations of the corrective action alternatives in relation to the corrective action objectives and screening criteria; and (5) Recommend a preferred corrective action alternative for the subsurface at CNTA. A Corrective Action Investigation (CAI) was performed in several stages from 1999 to 2003, as set forth in the ''Corrective Action Investigation Plan for the Central Nevada Test Area Subsurface Sites (Corrective Action Unit No. 443)'' (DOE/NV, 1999). Groundwater modeling was the primary activity of the CAI. Three phases of modeling were conducted for the Faultless underground nuclear test. The first involved the gathering and interpretation of geologic and hydrogeologic data into a three-dimensional numerical model of groundwater flow, and use of the output of the flow model for a transport model of radionuclide release and migration behavior (Pohlmann et al., 2000). The second modeling phase (known as a Data Decision Analysis [DDA]) occurred after the Nevada Division of Environmental Protection reviewed the first model and was designed to respond to concerns regarding model uncertainty (Pohll and Mihevc, 2000). The third modeling phase updated the original flow and transport model to incorporate the uncertainty identified in the DDA, and focused the model domain on the region of interest to the transport predictions. This third phase culminated in the calculation of contaminant boundaries for the site (Pohll et al., 2003).